An air strike in northern Afghanistan that killed up to 90 people hit at the heart of plans for a tactical change in the Western military strategy against Taliban-linked insurgents. The issue of civilian casualties in the NATO-backed war to rout militants from Afghanistan is a thorn in the relationship between the Kabul government and its Western backers -- and a simmering source of anger among Afghan people. Islamist insurgents in Afghanistan, Pakistan and elsewhere have become adept at manipulating battleground situations to draw fire and escalate civilian deaths to exploit local sensitivities about the presence of foreign troops. "They do it to fuel public dissatisfaction against the presence and military operations of foreign forces and it justifies their cause," Afghan lawmaker Ahmad Behzad said. NATO launched an air strike early Friday against Taliban militants who had hijacked two fuel tankers ferrying supplies for international troops, the International Security Assistance Force (ISAF) said. According to ISAF, insurgents were attempting to take the tankers across the Kunduz river, near the Tajik border, when one stalled and they called on villagers to help themselves to the fuel. Witnesses said as civilians were syphoning off fuel, the air strike ignited a fireball that killed dozens and caused horrific injuries to many others. Officials have said up to 90 people were killed, including a large number of militants, though exact figures are unclear. Investigations are under way into the cause and the breakdown of militant versus civilian deaths. The incident came as British Prime Minister Gordon Brown delivered a speech calling the Western strategy against insurgents in Afghanistan "flawed" yet essential to suppressing the worldwide terrorism threat. "It (the Kunduz strike) couldn't have come at a worse time for the Western powers trying to justify their presence in this country," said a foreign consultant in Kabul. "On the other hand, the timing is perfect for showing what international troops are up against," he said on condition of anonymity. A warning from President Hamid Karzai that targeting civilians was unacceptable highlighted a major point of contention between his government and its Western backers. Critics say Karzai has adeptly used the issue of civilian casualties in a bid to boost his waning domestic popularity, while ignoring the huge number of civilian deaths the Taliban cause directly. "By ignoring the Taliban role in inflicting civilian casualties in their direct operations, or by hiding among people to cause civilian casualties from foreign forces operations, the government has helped the Taliban in their propaganda against foreign forces and their presence," Behzad said. Nevertheless, air strikes have been singled out by commanders as a major cause of public dissatisfaction with the foreign military presence. The US and NATO chief in Afghanistan, General Stanley McChrystal, has ordered severe restrictions in an effort to reduce civilian deaths. European governments moved swiftly to urge an investigation into the air strike, with British Foreign Secretary David Miliband leading the charge, saying such incidents "undermine" NATO and Afghan commitment. "I look forward to a prompt and urgent investigation into what actually happened today," Miliband said. Washington voiced "great concern" over the loss of civilian life. The United Nations says nearly two-thirds of 828 civilians allegedly killed by pro-government forces in the conflict last year died in air strikes. Last month McChrystal issued counter-insurgency guidelines to troops that stressed the safety of civilians in the war against the Taliban, resurgent in recent months and seen extending their reach into previously peaceful areas. McChrystal has just submitted a review to his superiors of what he called a "serious" situation in Afghanistan, as the Taliban employ increasingly deadly tactics and remote-controlled bombs against foreign forces. These improvised explosive devices, or IEDs, have become the nub of the war, claiming the bulk of foreign troop casualties and also causing a high number of civilian deaths. "The (Afghan) government has helped the Taliban by ignoring civilian casualties caused by Taliban operations and the major role Taliban plays in civilian casualties caused by foreign forces," Behzad said. "The government has to address these issues, talk about it in the media and make the point clearly that it is the Taliban's presence in Afghanistan that causes civilian casualties."